Preheat oven to 350°F (175°C) and grease a 9x13 inch baking pan.

In a medium saucepan over medium heat, melt the butter and granulated sugar together.

Remove from heat and stir in eggs and vanilla extract until combined.

Add flour, cocoa powder, and salt, mixing until just combined.

Pour the batter into the prepared pan, spreading it evenly.

Bake for 20-25 minutes, or until a toothpick comes out with a few moist crumbs. Allow to cool completely.

Melt chopped chocolate in a heatproof bowl over simmering water.

In another bowl, whip heavy cream until soft peaks form.

Fold the melted chocolate into the whipped cream until fully combined.

Spread the mousse over the cooled brownies and refrigerate for at least 2 hours.

Heat remaining heavy cream until it simmers.

Remove from heat and whisk in chopped chocolate until smooth.

Allow to cool slightly, then pour over the mousse, spreading evenly.

Chill for an additional hour before slicing.